Filter medium

ABSTRACT

The present invention relates to a filter medium comprising a substrate and a fine fiber layer on top of the substrate, wherein the substrate comprises
     a first layer comprising first fibers having a first average diameter and a first maximum fiber length;   a second layer comprising second fibers having a second average diameter and a second maximum fiber length; and   a third layer comprising third fibers having a third average diameter and a third maximum fiber length;   wherein   the boundary area between the first and the second layer forms a first blended area comprising first and second fibers; and   the boundary area between the second and the third layer forms a second blended area comprising second and third fibers;   and wherein the first and the third average diameters are each larger than the second average diameter.

The present invention relates to a novel filter medium, a process forthe preparation of the medium, a filter package including the medium andthe use of the medium in a method of filtering a gas or a liquid.Specifically, the invention relates to a filter medium that is efficientin the removal of particulates without causing a significant pressuredrop.

There is an increasing need to filter impurities from the atmosphere,vapors and fluids. Specifically, air intake filtration in thetransportation area as well as in the field of gas turbines requiresmedia that are efficient in terms of particulate removal. Moreover,indoor air quality is becoming important as respiratory illnesses,allergy and asthma symptoms occur with increasing frequency inindustrialized civilization. While heating, ventilation and airconditioning (HVAC) filters can provide high particulate removalcapability, these filters also create significant air flow resistance.As a result, high efficiency HVAC filter systems require powerful fansto move air. Likewise, prior art media for air intake applications ingas turbines and transportation lead to an increased energy consumption.This goes along with tremendous CO₂ emission.

Prior art filter media use inorganic and synthetic fibers for thefiltration of, e.g. air. By definition, the filters' energy consumptionincreases over time because their resistance to air flow increases withthe amount of particulates which are removed from the air.

Unfortunately, prior art media show a significant increase in theirresistance to air flow as they fill up easily with dust and fineparticulates. When these particulates are removed from the air, theenergy consumption, therefore, increases.

To ensure sufficient particle removal, prior art suggests filter mediathat are made of multiple layers of different porosities. U.S. Pat. No.4,661,255 teaches the use of a multiple layer material wherein theporosity decreases in the direction of flow. That is, the individuallayers have increasingly fine or narrow pores. Such media, however, arenot satisfactory because small and bigger particulates tend to clog themedia very easily over time. As a consequence, air flow resistanceincreases quickly.

It is thus an object of the present invention to provide an improvedfilter medium that avoids the risk of early clogging. It is a furtherobject of the present invention to provide a filter medium which avoidsor at least reduces the pressure drop observed over time to thusminimize total average air flow resistance. It is another object of thepresent invention to avoid an early onset of an increasing pressuredrop.

In a first aspect the present invention therefore relates to a filtermedium as defined in claim 1. Preferably, the filter medium comprises asubstrate and a fine fiber layer on top of the substrate, wherein thesubstrate comprises

-   a first layer comprising first fibers of at least 80% synthetic    staple fibers having an average diameter of 6-45 μm, preferably 7-38    μm, most preferably 7-17 μm;-   a second layer comprising second fibers of at least 10% micro fibers    having an average diameter of 0.4-5 μm, preferably 0.6-2.6 μm; and-   a third layer comprising third fibers of at least 80% synthetic    staple fibers having an average diameter of 6-45 μm, preferably 7-38    μm, most preferably 7-17 μm;    wherein-   the boundary area, i.e. the area of contact, between the first and    the second layer forms a blended area comprising first and second    fibers; and-   the boundary area between the second and the third layer forms a    blended area comprising second and third fibers.

Staple fibers as used herein are short cut fibers that are not longerthan 45 mm.

Microfibers as used herein are fibers having an average diameter of0.4-5 μm and a maximum length of 45 mm, preferably 18 mm, morepreferably 10 mm, most preferably 3 mm.

It has been found that the filter medium described herein is capable ofremoving particulates in an efficient manner without causing asignificant increase in pressure drop. That is, the filter medium of thepresent invention exhibits a lower pressure drop at a given particleremoval efficiency compared to prior art media.

FIG. 1 shows typical pressure drop curves for a given efficiency forcommercially available filter media (MG media), for commerciallyavailable spunbond filter media with a nanolayer and for the product ofthe invention.

In a second aspect, the present invention relates to a process asdefined in claim 16. In a preferred embodiment, the process comprisesthe steps of

-   -   providing first, second and third homogeneous slurries;    -   supplying the first slurry onto a dewatering screen to form a        first deposit;    -   supplying the second slurry onto the first deposit to form a        second deposit on top of the first deposit;    -   supplying the third slurry onto the second deposit to form a        third deposit on top of the second deposit;    -   removing the water from the deposits to form a wet fibrous mat        or sheet;    -   drying the wet fibrous mat or sheet while heating to form a        substrate; and    -   applying a fine fiber layer on top of the substrate;        wherein

-   the first slurry comprises water and first fibers having at least    80% synthetic staple fibers an average diameter of 6-45 μm,    preferably 7-38 μm, most preferably 7-17 μm;

-   the second slurry comprises water and second fibers having at least    10% of microfibers, preferably microfibers having a diameter of    0.4-5 μm, most preferably a diameter of 0.6-2.6 μm; and

-   the third slurry comprises water and third fibers having a least 80%    synthetic staple fibers having an average diameter of 6-45 μm,    preferably 7-38 μm, most preferably 7-17 μm.

Third and fourth aspects of the invention relate to filter elementsincluding the filter medium and to the use of the filter medium in amethod of filtering a gas or a liquid, in particular in HeatingVentilation and Air Conditioning filters and Gas Turbine intake airfiltration applications.

DETAILED DESCRIPTION OF THE INVENTION

The filter medium of the present invention comprises a substrate and afine fiber layer on top of the substrate. In a preferred embodiment, thefine fiber layer is deposited on the first layer. The substratecomprises a first, a second, and a third layer.

Substrate

As used herein, the first layer of the substrate comprises first fibershaving a first average diameter and a first maximum fiber length.

The second layer comprises second fibers having a second averagediameter and a second maximum fiber length.

Likewise, the third layer comprises third fibers having a third averagediameter and a third maximum fiber length.

As used herein, the term first, second and third average diameterrelates to the average diameter of the first, second and third fibers,respectively. These first, second and third fibers are contained in thefirst, second and third layers, respectively.

In a preferred embodiment, the first, and the third fibers form thedominating material in the first, and the third layer, respectively. Inthis context, the term dominating material is to be understood such thatthe first, and the third layer contain significant amounts of the first,the third fibers, respectively. In a preferred embodiment, these layersessentially consist of the individual fibers. That is, the first layermay essentially consist of the first fibers; and the third layer mayessentially consist of the third fibers. In a most preferred embodiment,the first and third layer contains up to 80 wt. % of the first and thirdfibers respectively.

In a preferred embodiment, the second fibers are those having thesmallest average diameter of all fibers in the second layer. In apreferred embodiment the second layer contains at least 10 wt. %, mostpreferably at least 30 wt. %. of these second fibers having the smallestdiameter of all fibers in the second layer.

Each of the first, the second and the third fibers may be a mixture oftwo or more fibers. That is to say, the first fibers may be a mixture oftwo or more types of fibers; the second fibers may be a mixture of twoor more types of fibers; and the third fibers may be a mixture of two ormore types of fibers.

Between the first and the second layers there is a boundary area (hereinafter also referred to as first boundary area). This boundary area,which is the area of contact between the first and the second layer,comprises a mixture of the first and the second fibers and, thus, formsa blended area. If at least one of the first and the second fiberscomprises two or more sorts of fibers, then this blended area preferablycomprises all components that form the first and the second fibers.

Likewise, there is a boundary area between the second and the thirdlayer which area comprises a mixture of the second and the third fibers.Accordingly, this boundary area (herein after also referred to as secondboundary area) which is the area of contact between the second and thethird layer, forms a blended area of second and third fibers. Theblended area preferably comprises all components that form the secondand the third fibers. The second boundary area may have a smallerthickness than the first boundary area if the first boundary arearesults from first providing the first slurry onto the dewatering screento form the first deposit followed by supplying the second slurry ontothe first deposit as described with regard to the process within thisinvention.

The first and the third average diameters are larger than the secondaverage diameter. It is believed that this arrangement of fibers resultsin a substrate wherein the first and the third layers have a structureof higher porosity/openness than the second layer which is located inbetween the first and the third layer.

In a preferred embodiment, the average diameters of the first and thethird fibers are each independently of from 6-45 μm, preferably of from7-38 μm, and most preferably of from 7-17 μm.

In a further embodiment, the maximum fiber length of the first and thethird fibers (hereinafter also referred to as first and third maximumfiber length) independently from each other is up to 45 mm, preferablyup to 24 mm, and most preferably up to 12 mm.

Average fiber diameters and maximum fiber lengths can be determined byway of scanning electron microscopy (SEM) using a Phenom scanningelectrom microscope (available from FEI) integrated with Fibermetricsoftware (Phenom World, The Netherlands).

The first and the third fibers are independently from each otherselected from at least one of the group consisting of natural fibers,polymeric fibers and inorganic fibers. Preferably, the first and thethird fibers are independently from each other selected from at leastone of the group consisting of polymeric fibers and inorganic fibers.

Examples of natural fibers include fibers or derivatives of naturalfibers such as cellulose, lyocell, viscose or any other derivatives ofcellulosic fibers.

Polymeric fibers in the context of the present invention includepolyester, polyethylene, polyethylene terephthalate, poly butyleneterephthalate, poly amide, poly propylene, acrylic, bicomponent fibersof PET-CoPET, PE-PET, and the like. In a preferred embodiment, thepolymeric fibers are selected from the group consisting of polyethylene,polyethylene terephthalate, polypropylene, polybutylene terephthalateand polyamide.

If the first and/or the third fibers are selected from inorganic fibersor include such fibers, then the inorganic fibers are preferably glassfiber strands or basalt fibers. Synthetic fibers as used herewithin areany fibers that are man-made and include polymeric, cellulosic andinorganic fibers.

In a further embodiment, the invention relates to a filter mediumwherein the first fibers in the first layer and the third fibers in thethird layer independently from each other are present in an amount of atleast 65 weight % based on the total fiber weight of the first and thethird layer, respectively. In a preferred embodiment, the amount of thefirst and the third fibers in the first and the third layer,respectively, is at least 80 weight % or more.

The substrate of the filter medium comprises a second layer comprisingsecond fibers. These fibers have an average diameter (herein alsoreferred to as second average diameter). In a preferred embodiment, thesecond average diameter is of from 0.4-5 μm, preferably of from 0.6-2.6μm; and/or wherein the second maximum fiber length is 18 mm, preferably10 mm, more preferably 3 mm. The second fibers are preferably selectedfrom the group consisting of synthetic fibers and microglass fibers.Examples of synthetic fibers include polyester, polyethyleneterephthalate, poly butylene terephthalate, poly amide, poly propylene,acrylic, bicomponent fibers of PET-CoPET, PE-PET, and the like.Preferred examples of synthetic fibers are polyester, polyethylene,polyethylene terephthalate, poly butylene terephthalate, bicomponentfibers of PET-CoPET, PE-PET, and the like. Examples of microglass fibersare B Glass, C glass, both air attenuated and flame attenuated.Preferred examples of microglass fibers include borosilicate choppedglass fibers having a diameter of 0.6 and 1.0 micron (LFI, Germany).

In a preferred embodiment, the second fibers in the second layer arepresent in an amount of at least 10 weight %, preferably at least 20weight %, most preferably at least 30% based on the total weight of thesecond layer.

As described herein above, the average diameters of the first and thethird fibers (also referred to herein as first and third fibers) arelarger than the average diameter of the second fibers (second averagediameter).

Without wishing to be bound by theory, it is believed that this specificselection gives an arrangement of fibers wherein the second layer of thesubstrate comprises the smallest fibers within the substrate. It isbelieved that this arrangement translates into a structure wherein thepores between the fibers in the second layer are particularly small,whereas the first and the third layers have a higher porosity oropenness. Given that the first and the third average fiber diameters arelarger than the second average diameter, this profile can be describedas converging and diverging pore size. While it is undesirable to havesmall diameter fibers in the first and third layers, (small) amountsthat do not interfere with the converging-diverging pore structure ofthe media are considered within the scope of this invention. It isbelieved that such a converging and diverging pore size is of particularvalue for filtering particulates with lower pressure drops and thusenergy savings in HVAC applications and other filtering applications.

In a preferred embodiment of the invention, the ratio of the first andthe second average diameters (D_(1st)/D_(2nd)) and the ratio of thirdand the second diameters (D_(3rd)/D_(2nd)) are each independently fromeach other of from 1.2 to 115, preferably of from 1.4 to 43.

In a further embodiment of the invention, at least one of the first, thesecond and the third layer contains a further sort of fibers in additionto the first, the second and the third fibers. This further sort offibers can be understood as an additional structural type of fibers.That is, the first, the second and the third layer may contain otherfibers than the first, the second and the third fibers. Preferably, thisadditional sort of fibers is selected from binder fibers, preferablybicomponent fibers and glass fiber strands. In a most preferredembodiment, the first, the second and/or the third layer containsbicomponent fibers and glass fiber strands.

Bicomponent fibers are fibers usually containing more than one polymer.They can have a core and sheath structure.

Preferably, further sorts of fibers are contained in the first, thesecond and/or the third layer in an amount of from 2-70 weight % basedon the total fiber weight of the first, the second and the third layer,respectively.

In a further preferred embodiment, the first, the second and the thirdlayer are essentially free from natural fibers such as cellulose. It hasbeen found that filter efficiency and pressure drop characteristics areparticularly good during the filtration of gas such as air when thesubstrate contains no natural fibers that tend to attract moisture oroil or interact with salt from the surroundings. Without wishing to bebound by theory, it is believed that such natural fibers attract water.Moreover, fibers such as cellulose were found to bind oil and salt. Thismay lead to a swelling or clogging of the fibers which results in a lossof filter efficiency and also goes along with a significant increase inpressure drop.

Fine Fiber Layer

The filter medium of the present invention comprises a fine fiber layeron top of the substrate. In a preferred embodiment, the fine fiber layeris deposited on top of the first layer which is described herein above.In a further embodiment, the fine fiber layer adheres to the substrateof the filter medium, preferably to the first layer, by means of anadhesive. Accordingly, the filter medium preferably comprises anadhesive between the substrate, preferably the first layer, and the finefiber layer.

In a preferred embodiment, the fine fiber layer is deposited by anyprocess that forms sub micronic fibers on the side of the first layerthat was adjacent to the dewatering screen during the formation process.

The adhesive can be any adhesive that can be spray-coated, roll coated,rod coated, metered or otherwise applied onto the substrate. Preferably,the adhesive is selected from a water-based polymer dispersion selectedfrom the group consisting of polyurethane and polyacrylic basedadhesives. In a preferred embodiment, the adhesive is applied to thesubstrate in a manner such that it does not affect the permeability ofthe substrate. That is, the adhesive preferably is applied with a coatweight of less than 4 g/m².

In a further embodiment, the fine fiber layer is attached to thesubstrate of the filter media by lamination, hot melt adhesive,ultrasonic bonding or point bonding.

In yet another embodiment of the invention, the fine fiber layer that isapplied to the substrate can be deposited on a further layer (hereinafter also referred to as carrier and/or support) before beingtransferred to the substrate comprising the first, the second and thethird layer. In this embodiment, the carrier preferably comprises fiberssimilar to those forming the dominating material in the layer of thesubstrate that is adjacent to the carrier/support. The term “similar tothose forming the dominating material in the layer of the substrate” inthe context of the present invention means that the carrier/support andthe substrate layer adjacent to it both comprise fibers which aresimilar with regard to its raw material, average diameter and/or maximumfiber length. In a particularly preferred embodiment, the fibers in thecarrier/support and the substrate layer adjacent to it are selected suchthat the fibers forming the dominating material in the respective layerhave a similar average diameter. In a further or different embodiment ofthe invention, the first and/or third fibers may be cut fibers having amaximum fiber length, whereas the carrier/support may comprisecontinuous fibers with a relatively higher length, wherein both types offibers are made from the same type of polymer.

When the fine fiber layer is deposited on a carrier/support, the finefiber layer which is applied on top of the substrate thus contacts thesubstrate via an intermediate layer. This intermediate layer does notamend the overall arrangement within the substrate wherein the averagediameters of the first and the third fibers are larger than the averagediameter of the second fibers. As explained herein above, selecting theaverage diameters of the first and the third fibers to be larger thanthe second fibers gives an arrangement of fibers wherein the secondlayer of the substrate comprises the smallest fibers within thesubstrate. This arrangement is believed to translate into a structurewherein the pores between the fibers in the second layer areparticularly small whereas the first and the third layers have a higherporosity or openness. This profile of individual average fiber diametersdoes not change when the fine fiber layer is applied to the substratewith the aid of a carrier and/or support wherein the carrier/supportessentially consists of a material similar to the material forming thematerial of the first or the third layer whichever is in contact withthe carrier/support. That is to say, the dominating material within thecarrier/support may in one embodiment of the invention be identical tothe first or the third fibers that form the dominating material in thefirst, and the third layer, respectively. In this context, theexpression “preferably identical” relates to the raw material formingthe fibers, to the average diameter and/or to the maximum fiber length.In a particularly preferred embodiment of the invention, thecarrier/support essentially consists of the same fibers as the first orthe third layer.

In such an embodiment, the first or the third layer within the substratethat is in contact with the carrier/support can thus equally be viewedat as forming a modified first or third layer (herein after alsoreferred to as first' layer or third' layer). When looking at the filtermedium from such a perspective, it is readily understood that thecarrier/support can also be defined as a fourth, i.e. separate layerwithin the substrate. Whether or not the carrier/support is consideredas part of a first' or third' layer or, alternatively, as a fourth layerwithin the substrate, is not so important as long as the carrier/supportdoes not amend the overall fiber arrangement wherein the pores betweenthe fibers in the second layer are particularly small whereas the first'and the third' layers have a higher porosity or openness. When using acarrier/support to attach the fine fiber layer to the substrate, it isthus readily understood that the carrier/support providing the contactbetween the fine fiber layer and the substrate preferably does notcontain small diameter fibers in an amount that would interfere with theconverging-diverging pore structure of the substrate. Using acarrier/support to attach the fine fiber layer to the substratetherefore does not alter the converging and diverging pore size whichwas found to be of particular value for filtering particulates withlower pressure drops and thus energy-savings in HVAC applications andother filtering applications.

The fine fiber layer comprises fine fibers having an average diameter upto 3 μm. In a preferred embodiment, the fine fiber layer comprisessub-micronic fibers. Sub-micronic fibers in the context of the presentinvention are defined by an average fiber diameter of below 1 μm. In apreferred embodiment, the fine fiber layer, therefore, comprises fibershaving an average fiber diameter of from 0.08-0.8 μm, preferably of from0.1-0.4 μm.

In a preferred embodiment, the sub-micronic fibers are polymeric fibers.Preferably, these fibers are selected from the group consisting ofpolyamide, polyester, polyethylene terephthalate, polypropylene,polycarbonate, polyurethane, polybutylene terephthalate andpolyvinylidene fluoride fibers. In a further embodiment, the fine fiberlayer entirely consists of these sub-micronic fibers.

In a further embodiment, the filter medium comprises a protective layeron top of the fine fiber layer. That is, the fine fiber layer ispreferably covered by a protective layer. The protective layer can beany type of layer that is preferably selected from the group consistingof polyester, polyethylene, polyethylene terephthalate, poly butyleneterephthalate, polypropylene, polyamide or any combination of the abovefibers and combinations hereof with PE-PET or PET-coPET bicomponentfibers.

It has been found that the filter medium described herein has excellentmechanical properties. In particular, the present medium shows a veryhigh bending stiffness of at least 500 mg as obtained from its bendingresistance according to TAPPI Standard T 543 (Gurley-type tester). Themedium can also be pleated and can maintain its shape withoutsignificant deformation during use. Moreover, the medium withstandsback-pulsed applications wherein the flow of the fluid (i.e. a liquid ora gas) is reversed to effectively clean the medium. In back-pulsed jetapplications, the fine fiber layer is on the upstream side of the media.The medium also has a high pore volume within the substrate which allowsfor high permeability values. Preferably, the air permeability of theinventive medium is at least 5 cfm as measured according to ASTM D737,for an area of 38 cm² and a pressure of 125 Pa. It has been furtherfound that the filter medium according to the invention has a filtrationefficiency of more than 35% against 0.4 micron particles at a facevelocity of 5.3 cm/s when tested using a Palas MFP-30000 Filter TestSystem (Palas GmbH, Karlsruhe, Germany) connected to a REG 1000 dustfeeder according to EN 779 test method.

Surprisingly, the medium shows a reduced pressure drop over time ascompared to prior art filter media, that have similar mechanicalproperties and particle removal characteristics.

Process for the Preparation of the Filter Medium

The process for the preparation of the filter medium preferablycomprises the steps of

-   -   providing first, second and third homogeneous slurries;    -   supplying the first slurry onto a dewatering screen to form a        first deposit;    -   supplying the second slurry onto the first deposit to form a        second deposit on top of the first deposit;    -   supplying the third slurry onto the second deposit to form a        third deposit on top of the second deposit;    -   removing the water from the deposits to form a wet fibrous mat        or sheet;    -   drying the wet fibrous mat or sheet while heating to form a        substrate; and    -   applying a fine fiber layer on top of the substrate;        wherein

-   the first slurry comprises water and the first fibers described    herein above having at least 80% synthetic staple fiber having a    diameter of 6-45 μm;

-   the second slurry comprises water and the second fibers described    herein above having at least 10% micro fibers having an average    diameter of 0.4-5 μm; and

-   the third slurry comprises water and the third fibers described    herein above having at least 80% synthetic staple fiber having a    diameter of 6-45 μm.

In this process, first, second and third homogenous slurries areprovided. These slurries can be provided by any method known in the artsuch as by adding and mixing the fibers in water.

As used herein, the first homogeneous slurry comprises water and thefirst fibers preferably having at least 80% synthetic staple fiberhaving a diameter of 6-45 μm. Likewise, the second slurry compriseswater as well as the second fibers preferably having at least 10% microfibers having an average diameter of 0.4-5 μm. Moreover, the thirdslurry comprises water and the third fibers preferably having at least80% synthetic staple fiber having a diameter of 6-45 μm.

Once the first, the second, and the third homogeneous slurries areprepared, they are applied onto a dewatering screen. This screen can beany screen commonly used in a paper making process. Preferably, thisscreen is a dewatering endless screen. Upon supplying the first slurryonto the dewatering screen, a first deposit is formed on the screen.Subsequently, the second slurry is supplied onto the first deposit toform a second deposit on top thereof. Then, the third slurry is suppliedonto the second deposit to form a third deposit on top of the seconddeposit. Supplying the first, the second and the third slurries can becarried out by using different channels of a headbox of a wetlaidforming machine. Alternatively, the slurries can be applied one afterthe other using an dynamic handsheet former from Techpap SAS (Grenoble,France).

During or after deposition of the individual slurries, water is removedto form a wet fibrous mat or sheet. Subsequently, the wet fibrous mat orsheet is dried while heating to form the substrate. This substrate—asdefined herein above—comprises the first, the second, and the thirdlayer comprising the individual fibers.

It is believed that applying the first, the second, and the thirdslurries on top of each other results in a boundary area betweenadjacent layers which forms a blended area comprising first and secondor third and second fibers.

Within the boundary area, the components of the first and second and thecomponents of the third and second layers intermingle with each othersuch that there is a fibrous interlock rather than a sharp and definededge which would separate the individual layers from each other.

Without wishing to be bound by theory, the inventors believe that thisboundary area results in a smooth or semi-continuous gradient whereinthe fiber average diameter of the fibers that are contained in thesubstrate decreases from the outer layers to the mid-layer of thesubstrate. It is believed that this fiber arrangement results in aconverging-diverging pore size distribution which may be regarded asconverging and diverging pore size across the thickness of the filtermedium. This arrangement was found to be particularly valuable infiltering applications.

It is believed that unlike conventional media with a decreasing poresize gradient, this unique media structure allows for the early captureof large particles in the upstream region of the filter media, smallerparticles being trapped in the converging region, followed by a decreasein the face velocity of remaining particles as they enter the divergingregion. This reduction in face velocity and increasing pore structureallows for a better distribution of the remaining particles across thefine fiber layer as the fluid exits the filter, thus having a lowerimpact on the pressure drop.

Accordingly, the filter medium of the present invention is preferablyused in a way such that the fluid to be filtered exits the filter mediumat the side of the substrate upon which the fine fiber layer isallocated.

1-20. (canceled)
 21. A filter medium comprising a substrate and a finefiber layer on top of the substrate, wherein the substrate comprises afirst layer comprising first fibers having a first average diameter anda first maximum fiber length; a second layer comprising second fibershaving a second average diameter and a second maximum fiber length; anda third layer comprising third fibers having a third average diameterand a third maximum fiber length; wherein the boundary area between thefirst and the second layer forms a first blended area comprising firstand second fibers; and the boundary area between the second and thethird layer forms a second blended area comprising second and thirdfibers; and wherein the first and the third average diameters are eachlarger than the second average diameter.
 22. The filter medium accordingto claim 21, wherein the ratio of the first and the second averagediameters (d1st/d2nd) and the ratio of the third and the second averagediameters (d3rd/d2nd) are each independently from each other of from 1.2to 115, preferably of from 1.4 to
 43. 23. The filter medium according toclaim 21, wherein the first and the third average diametersindependently from each other are of from 6-45 μm, preferably of from7-38 μm, most preferably of from 7-17 μm; and wherein the amount offibers having a diameter of less than 5 μm within the first and thethird layer is less than 3 wt. %, preferably less than 1 wt. % and mostpreferably 0 wt. % based on the total weight of each of the first orthird layer and/or wherein the first and the third maximum fiber lengthindependently from each other is up to 45 mm.
 24. The filter mediumaccording to claim 21, wherein the second average diameter is of from0.4-5 μm, preferably of from 0.6-2.6 μm; and/or wherein the secondmaximum fiber length is 18 mm, preferably 10 mm, more preferably 3 mm.25. The filter medium according to claim 21, wherein the first and thethird fibers independently from each other are selected from the groupconsisting of polymeric fibers and inorganic fibers, wherein thepolymeric fibers are preferably selected from polyester, polyethylene,polyethylene terephthalate, polyolefin, polybutylene terephthalateand/or polyamide, and wherein the inorganic fibers are preferably glassfiber strands.
 26. The filter medium according to claim 21, wherein thefirst fibers in the first layer are present in an amount of at least 65wt. %, preferably at least 80 wt. % based on the total fiber weight ofthe first layer; and/or wherein the third fibers in the third layer arepresent in an amount of at least 65 wt. %, preferably at least 80 wt. %based on the total fiber weight of the third layer.
 27. The filtermedium according to claim 21, wherein the first, the second and/or thethird layer further contains up to 50 wt. %, preferably from 2-35 wt. %,more preferably 2-25 wt. % of binder fibers based on the total weight ofeach of the first, the second and/or third layer.
 28. The filter mediumaccording to claim 21, wherein the second fibers are selected from thegroup consisting of synthetic fibers and microglass fibers.
 29. Thefilter medium according to claim 21, wherein the second fibers in thesecond layer are present in an amount of at least 10 wt. %, preferablyat least 20 wt. %, more preferably at least 30 wt. % based on the totalweight of the second layer.
 30. The filter medium according to claim 21,wherein the fine fiber layer on top of the substrate comprises finefibers having an average fiber diameter of less than 3 μm.
 31. Thefilter medium according to claim 21, wherein the fine fiber layer on topof the substrate comprises sub-micronic fibers having an average fiberdiameter of less than 1 μm; preferably of from 0.08 to 0.8 μm, mostpreferably of from 0.1 to 0.4 μm.
 32. The filter medium according toclaim 21, wherein the fine fiber layer is deposited on top of the firstor third layer.
 33. The filter medium according to claim 21, wherein thefilter medium comprises an adhesive between the substrate and the finefiber layer.
 34. The filter medium according to claim 31, wherein thesub-micronic fibers are polymeric fibers that are preferably selectedfrom the group consisting of polyester, polypropylene, polyurethane,polycarbonate, polyamide and polyvinylidene fluoride sub-micronicfibers.
 35. The filter medium according to claim 21, comprising aprotective layer on top of the fine fiber layer.
 36. A process for thepreparation of a filter medium according to claim 21, the processcomprising the steps of a. providing first, second and third homogeneousslurries; b. supplying the first slurry onto a dewatering screen to forma first deposit; c. supplying the second slurry onto the first depositto form a second deposit on top of the first deposit; d. supplying thethird slurry onto the second deposit to form a third deposit on top ofthe second deposit; e. removing the water from the deposits to form awet fibrous mat or sheet; f. drying the wet fibrous mat or sheet whileheating to form a substrate; and g. applying a fine fiber layer on topof the substrate; wherein the first slurry comprises water and firstfibers having a first fiber average diameter and a first maximum fiberlength; the second slurry comprises water and second fibers having asecond fiber average diameter and a second maximum fiber length; and thethird slurry comprises water and third fibers having a third fiberaverage diameter and a third maximum fiber length; and wherein the firstand the third average diameters are each larger than the second averagediameter.
 37. A filter element including the filter medium according toclaim
 21. 38. Use of the filter medium according to claim 21 in a methodof filtering a fluid, in particular in heating ventilation and airconditioning filters and gas turbine intake air filtration applications.39. Use of the filter element according to claim 37 in a method offiltering a fluid, in particular in heating ventilation and aircondition filters and gas turbine intake air filtration applications.